Internal combustion engines produce undesirable induction noise within a vehicle. While the induction noise is dependent on the particular engine configuration and other induction system parameters, such noise is caused by a pressure wave that travels toward the inlet of the air induction system. Induction noise is particularly problematic in hybrid vehicles, as changes in ambient noise are particularly noticeable, particularly because engines in hybrid vehicles repeatedly turn on and off. Moreover, hybrids tend to operate a specific engine RPMs that maximize efficiency since the engine speed is not directly related to vehicle speed and can be varied by changing the generator speed (depending on the powertrain architecture).
To address such noise, it is known to utilize exhaust mufflers to reduce engine exhaust noise, as well as smooth exhaust-gas pulsations. Some known mufflers include a series of fixed expansion or resonance chambers of varying lengths, connected together by pipes. With this configuration, the exhaust noise reduction is achieved by the size and shape for the individual fixed expansion chambers. While increasing the number of channels can further reduce exhaust noise, such configurations require additional packaging room within the vehicle, limiting design options for various components. Further, while mufflers traditionally include sound deadening material, such material only dampens sounds over a broad band of higher frequencies.
Another proposed solution for addressing undesirable noise is use of a Helmholz resonator or a quarter-wave resonator. These resonators produce a pressure wave that counteracts primary engine order noise waves. Such resonators consist of a fixed volume chamber connected to an induction system duct by a connection or neck. However, such arrangements attenuate noise only at a fixed narrow frequency range.
However, the frequency associated with the primary order of engine noise is different at different operating levels. Thus a fixed geometry resonator would be ineffective in attenuating primary order noise over much of the complete range of engine speeds encountered during normal operation of a vehicle powered by the engine. Moreover, such conventional resonator systems provide an attenuation profile that does not match the profile of the noise and yields unwanted accompanying side band amplification. This is particularly true for a wide band noise peak. The result is that when a peak value is reduced to the noise level target line at a given engine speed, the amplitudes of noise at adjacent speeds are higher than the target line. While multiple resonators could be used to address different frequencies, such a solution requires additional packaging room within a vehicle.
While not as common as the passive devices described above, active noise cancellation systems have also been employed in vehicle exhaust systems. Active noise cancellation systems include one or more vibrating panels (i.e., speakers) that are driven by a microprocessor. The microprocessor monitors the engine operation and/or the acoustic frequencies propagating in the exhaust pipe and activates the panels to generate sound that is out-of-phase with the noise generated by the engine to minimize or cancel engine noise. The principle is similar to that used by noise-canceling headphones. However, active devices have significant drawbacks. Some active devices are positioned within a cab of a vehicle and thus require sufficient packaging room for positioning, while maintaining an aesthetics. Other active devices have been placed in the automotive exhaust systems. However, in these arrangements, the microphones and speakers must be more powerful and capable of withstanding the intense heat and corrosive environment of an automobile exhaust. Furthermore, active devices are often cost-prohibitive for many vehicles.
A noise attenuation device that is capable of variable frequency noise reduction is needed.